Hey y'all. Been gone a long time. Hope you can help me:A residential client wants an isolated ground for optimal performance in his audio/visual entertainment system. I plan to run 12-3 from isolated-ground type recept, marking the red wire as green ("clean" ground) and attaching it to isolated groundscrew on device, then running the 12-3 into branch circuit panel, but with the (red) isolated ground wire continuing on to the service entrance for bonding (i.e., no bonding at branch circiut panel). My question is: what do I do with the other ground ("dirty", uninsulated copper)? As this is a residential job, all boxes are plastic and there is no conduit. At the panel I will of course terminate this bare copper on the equipment grounding bar; but what do I do at the recept? Since on an isolated-ground recept the yoke is isolated from the device's grounging screw, can I (now this is gonna sound like a real hack, but...)..can I wrap the bare copper around the screw that attaches the yoke to the plastic box? Is there a better (i.e., more proper) way?

An isolated ground is simply a ground wire that runs straight from the device to the panel, without connection to other ground wires along the way. Since you're running a dedicated circuit, the usual ground wire will do just fine. Heck, with plastic boxes, you don't even need a fancy IG receptacle. The yoke is never in contact with anything that's also grounded. Just connect the ground wire to the screw on the receptacle.

Thanks for the quick reply. But still I have these concerns: NEC 250.146(D) "Isolated Receptacles" requires "an insulated equipment grounding conductor" which I won't have with 12-2 romex since the copper is bare. The same article requires that this isolated ground terminate only at the service, i.e., not in the branch circuit distribution panel. Further thoughts?

What John is saying is you don't need IG hardware if you run a dedicated circuit in a plastic box. The ground is "isolated" simply because there are no other connections.

The NEC is simply recognizing that IG is really designed for places that use metal raceways that are also incidentally grounded a number of places in the building, creating ground loops.IBM decided in the late 80s that IG was snake oil and removed the requirement from the physical planning manual. Unfortunately a lot of people who should know better did not get the word.

Thanks for the quick reply. But still I have these concerns: NEC 250.146(D) "Isolated Receptacles" requires "an insulated equipment grounding conductor" which I won't have with 12-2 romex since the copper is bare. The same article requires that this isolated ground terminate only at the service, i.e., not in the branch circuit distribution panel. Further thoughts?

1. It's going to have to use MC-HCF (for Health Care Facilities) or some other form of IG cable anyway. The armor of MC cable doesn't meet the requirements for grounding. It's both the insulated ground in conjunction with the armor that's required for grounding path. An additional ground wire (present in the IG cable) is needed for the IG circuit.

2. It's going to have to have a metal box, now that you're using MC cable. Plastic boxes can't be used with MC cable

Just keep in mind if you do decide to run IG the green insulated wire will start at ground unity, ( the ground rod ) and run isolated from there to any device on the IG circuit. True IG installs are expensive.

The IG should terminate on the bus where the main bonding jumper lands according to the old standards IBM used. Then it passes through all cabinets on the load side without connection to the EGC or enclosure. You can create a "tree" system on insulated bus bars radiating from the MBJ bar.

All that said, IBM decided there really wasn't much to be gained by it and removed the recommendation from our books.

All that said, IBM decided there really wasn't much to be gained by it and removed the recommendation from our books.

Well, yah, if you try to build a network using RS-232 or using some sort of coax-cable signaling that relies on the shield being a common ground, you're pretty much toast if you have ground noise or ground differentials.

Modern signaling systems like Ethernet use coupling transformers with minimum 2000V isolation. Ethernet doesn't really care if you have ground problems.

I note that the guys at Xerox PARC found about a 90V ground differential between one end of the building and the other when they installed their first experimental Ethernet network.